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X-ray detection with films

The very first "detector" type that was already available at the time of the discovery of X-rays is film. The material constituents are different sorts of silver halide particles (AgBr, AgCl, AgI etc.) residing in an emulsion in a form of a thin surface layer (e.g. gelatine). These compounds are very unstable and already a very low intensity light can cause disintegration:

Ag^{+}Br^{-}\overset{h\nu }{\Rightarrow }Ag+Br\; \; \; \; \; \; (25)

The carrier gel keeps the silver halide stable, i.e. it blocks reduction. The silver becoming neutral form small grains in the emulsion with magnitude proportional to the intensity and spectral composition of light. In unexposed film grain sizes are in the range of 0.1 – 1 µm depending on film quality. The films have to be developed, i.e. the free Ag distribution has to be made visible, further the layer should be fixed for protection against mechanical and chemical effects. The emulsion layer is applied on a polyester basis with further protective and sensitivity intensifying layers: for increasing mechanical stability, for increasing sensitivity, for decreasing reflectance, etc. As the layer containing the light sensitive grains can only absorb 1-3% of the incoming X-ray flux, a further layer is set in front, called the image intensifier. Usually this is a phosphor containing layer that emits visible light and this light can be absorbed with a much higher efficiency.
An important concept in roentgenology is the density, the logarithm of the measure of the "blackening" caused by the X-ray radiation. This is an empirical function aiding the comparison of film types regarding sensitivity. The film density behavior in response to illumination can be seen in Fig. 14.

Fig. 14. Typical film illumination-density curve

The density-illumination curve can be experimentally determined by operating the X-ray device at a certain anode potential and changing the anode current, thus changing the X-ray flux as the number of emitted photons are proportional to the anode current. Another method is to put a set of Aluminum or iron absorber layers with known thicknesses in front of the film, resulting in varying degree of blackening proportional to the thickness. That will allow for constructing an empirical function.
Another important concept when describing films is contrast, a function constructed of the maximum and minimum density points (brightest and darkest) (26), several formulas are in use.

K=\frac{I_{max}-I{min}}{I_{max}+I{min}}\; \; \; \; \; \; or\; \; \; \; \; K=\frac{I_{max}-I{min}}{I_{max}}\; \; \; \; \; \; (26)

The greater the brightness difference of points of the image the higher the contrast is. This also depends on certain parts of the object to project and on the spectral properties of the X-ray beam. If the absorption variations in the projected object are significant, the image will show high density differences, thus will show high contrast. Contrast can be influenced by three physical factors: 1, the material of the anode of the X-ray tube, 2. the anode-cathode voltage, 3. the selective filtering of the X-ray beam.


Fig. 15. Images taken with anode voltage of 80kV (a) and 100 kV (b)

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